Carvin Nomad 112 Bias Question

[Perkinsman]

I found that there are 2 methods of biasing this unit, both currently on the web and similar but not the same. The one titled "Nomad Bias" is measuring (and adjusting) for 75ma at the standby switch but the other, titled "Vintage Series Bias" is a plate dissapation method. I found that in order to get the recommended 70% of max el84 plate dissapation that I had to raise the current at the standby switch to approx 92ma.

Is there actually a difference between the Vintage series or is one of the above bias methods incorrect?

[Enzo]

Neither is incorrect. What is incorrect is the idea that the bias is some super critical precise adjustment. You can bias an amp so hot the tubes get red hot from excessive current. What we call red-plating. And you can bias an amp so cold the crossover distortion is too high to listen to. But really, in between those extremes, the amp doesn't really care. You may prefer the sound at some particular setting, but that is a matter of taste, not correctness.

My reaction to those methods you found: Nomad bias means they liked the performance at 75ma, assuming average tubes, regardless of what the dissipation worked out to. The vintage method sounds like an invitation to use 70% or some other target you like as the goal, regardless of the current needed to achieve it. IN general, guys bias their amps a lot hotter than amp makers intend. For example Fender didn't ship amps biased to 70%. SO Carvin is telling you to either set it for their level, or pick a dissipation target and use that. That acounts for the one being hotter than the other.

I don't recall the model, but Carvin had one a while back that was adjusted by mains current draw.

[Perkinsman]

That ties alot of loose ends together about what I've thought about biasing, thanks. I'd always thought that there is a specific number to adjust bias current to & anything higher or lower is not in spec, which led me to believe that there was only one way to bias. Sothen if I understand correctly now, regardless of current or dissipation, as long as the max rated dissipation isn't exceeded, and the current is enough to turn the tube on, it's entirely up to the players preference, but the manufacturer provides the average player wanting to extend tube life a recommended current of 70% of the output tubes max diss current. Btw, is Fenders factory bias setting below or above 70%?

[Enzo]

Your typical Fender amp is cooler than 70% as shipped.

In my experience, the super gainy Peavey 5150 sits at about 11-15ma idle current. SO maybe 7-8 watts, so about 25%.

[Perkinsman]

Very interesting, I wonder if all mfr's ship at below 70% because they're trying to extend tube life or just collectively think it's more appealing to the masses. Will cooler tubes clip the same as high bias or do they sound more brittle?

[Enzo]

HAving discussed it with Peavey, for years they made their amps not adjustable. They designed amp with the tone coming from the preamp, and expected the power amp to be clean, loud, and reliable. They like others had found that make it adjustable, and folks would turn it way up and experience shorter tube life, and less reliable operation. LAter, like the 5150-2, they made it adjustable, because peeople just HAD to have adjustable...marketing, y'know, but with limited range, so you couldn't get in trouble if you fiddled. They even said, "Adjust it by ear". Mesa had similar views.

Fender? Look at the very popular Hot Rod DeVille. The one ohm cathode resistor is shared by the pair of power tubes, and has a adjustment spec of 60mv. That means 30mv per tube. B+ is 485v, so dissipation would be. About 15 watts, or 50% for a 6L6GC.

They ship them cooler than 70%, because "70%" is an internet thing anyway. It does make for longer tube life. And that means better reliability. If your tubes blow up, the amp quits. WHen the amp quits, the owner thinks "Damned Peavey (or Fender or Marshall, or whatever) isn't reliable. They never think, "Geez, I cranked my tubes up to almost melting."

I have no idea about brittle. AMps have such a wide range of response to tube characteristics, and tubes them selves vary A LOT in their characteristics. That is why they sell "matched sets", because individual tubes can vary so much. When Groove Tube or whoever had ten levels of whatever it was they matched, it wasn't that they made ten kinds of that tube. It was that that tube naturally came out at so many levels they could sort them.

As to marketing appeal, if you walk into the store and play a new amp, you are thinking about its tone, not its idle percentage.

Now if you like hot biased tubes for the sound, great. That is fine. Some guys have reported their amp sounded at its very best ever as the tubes were cherry red and about to fail. MAny guys had no idea their 5150 was biased so cold, until the internet told them so.

These are my opinions, others may disagree with me.

[Mick Bailey]

I always bias on the cool side to prolong tube life and when amps go out they get played and everyone is happy. Given the price of tubes in the UK and the short warranty I don't think anyone would argue. Most of the time what people hear from their amps is masked by numerous pedals and what they're looking for is clean volume. The upside of biasing cool is also the background noise reduction at idle.

Usually I don't specifically mention where the bias is set, but I did a repair job recently where the amp had the same tubes since 2003. The customer had no money to buy another set and asked me to return the amp with the old ones - something I really didn't want to do. Anyhow, he used the amp and I asked him how it was going and he said it was fine, but should he worry about the tubes. I said yes, but to give them a better chance I've biased the amp on the cool side until he got some cash together. I later got a call saying he'd researched this on the web and because the amp was biased cool the tone controls didn't work as they used to and the amp has no volume. It's a 100W amp and I have it on my bench today and it makes 94W on the old tubes.

The point is everything was fine until I mentioned biasing.

[Perkinsman]

Great thread, I'm glad to have joined this forum. For giggles and to expand my personal knowledge, I tried biasing hot & cold....to be honest, I couldn't hear much audible difference in tone although the hotter bias gave a bit more volume....it would never be heard in a live band mix situation.

[Leo_Gnardo]

I later got a call saying he'd researched this on the web and because the amp was biased cool the tone controls didn't work as they used to and the amp has no volume. It's a 100W amp and I have it on my bench today and it makes 94W on the old tubes.

The point is everything was fine until I mentioned biasing.

No, everything was fine until Mister Shortpockets started reading disinformation from dubious "experts" on the interweb.

Plenty of times I've installed new tubes and come up with less than 100W. Other times more. Of course I don't need to tell you, his tubes are old and shagged out. And they sound it, apparently.

Might give this guy the midrange notch mod, the one where you short 2 pins of the mid pot together. Takes 1 cm of wire. That "seems" to make the controls do more, for those seeking that sort of thing.

[Mick Bailey]

I have scoped amps to see how biasing affects output level and for the ones I've checked the difference is very slight - too small in my opinion to make any difference within the normal range of operation.

Edit; in reply to perkinsman

[Enzo]

I tried biasing hot & cold....to be honest, I couldn't hear much audible difference

Many share that experience. I believe some folks get differences they hear and have preferences, but if all is pretty much equal for your ears, then set it at the colder setting, and your tubes will last longer.

For giggles and to expand my personal knowledge,

Never be afraid to try something and learn from it. And never let anyone tell you different. And most of all, never take advice from the internet...

[Enzo]

The difference between 50 watts and 100 watts is only 3db. Whatever tiny power differences may occur will be barely audible at best.

I suspect the main differences we hear is a bit of gaininess in the power stage.